since i have traveled on both lines relatively recently, i thought i'd add my $0.02. im also in my mid-twenties, am very active (play lots of sports), but also enjoy relaxing around the pool with a drink and going to the bars/club:

overall, both are very similar, and you will have a fantastic time on either...there are a few minor differences, but neither is really worth making a big deal over (like others said, make your decision on price and itin.)

but, since you mentioned "adventure", there is one thing i want to point out - i felt that there were more "active" things to do on RCCL (and i'm refering to the Adventure of the Seas)...though both the Carnival Miracle and the Adventure had sports decks, RCCL had a bunch of sports tournaments (volleyball, dodgeball, soccer, basketball, etc.) which Carnival did not have....also, RCCL had the rockwall - you could do normal climbing, or speed climbing, or other variations on regular climbing...lots of ships also have skating (ice and inline), and some have the flow rider (surfing)

i did enjoy carnival's food a bit better - but i certainly didnt starve on RCCL